Ok the right answer is cotton gauze.
While the OEMs overwhelmingly opt for paper air filters, cotton gauze filters are a staple of aftermarket induction systems. So what makes the cotton gauze medium so much more conducive to airflow? “Paper filters are very inexpensive to manufacture, but by nature, the paper media is very restrictive. Cotton gauze allows for much more airflow, but it also presents the risk of more contaminants passing through the filter itself,” McGee explains. To combat this, Airaid applies a layer of oil to the cotton during the manufacturing process so that any stray contaminants will get trapped by the oil. “The result is a substantial increase in airflow over a stock filter without compromising the filtration abilities. Airaid takes things one step further by adding a synthetic layer on top of our four cotton layers. The synthetic layer prevents oil from getting sucked into the motor and wreaking havoc on the mass airflow sensor.”
Read more:
How Air Filters Work - Chevy High Performance Magazine